A Lewis Acidity Scale in Relation to Rate Constants of Lewis Acid Catalyzed Organic Reactions

The quantification of Lewis acidity is accomplished by a deuterated quinolizidine probe utilizing H-2 NMR spectroscopy. The chemical shifts of the H-2 NMR signals for aluminum, boron, titanium, and zinc halides are reported. The rate constants for Lewis acid catalyzed reactions were determined by utilizing UV/Vis spectroscopy for Diels-Alder and Povarov reactions under pseudo-first-order conditions. The magnitude of the H-2 NMR chemical shifts correlates with the rate constants of the organic transformations for many Lewis acids investigated in the same order, whereas some deviations are identified.